BOSTON — Measles, a preventable disease that has been largely vanquished in the United States, continues to show up sporadically in the population as unvaccinated people traveling to other countries unwittingly bring back infections, researchers report. Data released at a meeting of the Infectious Diseases Society of America show that 2011 has been a bad year for this kind of spread, with the highest number of U.S. measles cases since 1996.